1. Field of the Invention
The invention relates to a capacitive distance sensor having an elongated sensor element, especially for application in the automobile industry.
2. Description of the Background Art
Capacitive distance sensors are used for detecting an object approaching a vehicle or vehicle part, for example as part of a parking aid or as part of a contactless clamping protection for electrically adjustable windows or doors. A further field of application for such capacitive distance sensors is in space travel or in the use in industrial plants in order to be able to determine the presence of persons and/or objects and optionally the (quantitative) distance of these persons or objects from the sensor in a contactless manner.
A capacitive distance sensor having a sensor element enclosing a sensor area and mounted on an element to be protected is known from US005166679A. The sensor area is essentially a planar, extended, thin and electrically conductive area and forms one electrode of a capacitor. An object penetrating into the area to be monitored by the sensor serves as the counterelectrode of the capacitor. In the case of an approach of the object to the sensor element, the capacitance of the capacitor formed between the sensor area and the counterelectrode changes. The change in capacitance is measured directly and/or indirectly by means of electronics and from this the distance of the object from the sensor element is determined.
It is disadvantageous in this sensor arrangement that a relatively large area of the element to be protected is covered by the sensor area.
In DE 10 2008 031 307 A1, a further capacitive distance sensor is disclosed, the sensor element of which comprises a coaxial cable. The core of the coaxial cable is used as sensor area with the aid of which the approach of an object to the sensor element is monitored. The outer conductor of the coaxial cable is used as shielding and is removed at certain locations so that only a predefined spatial area is monitored by the sensor element.
In this embodiment, the comparatively small sensor area is disadvantageous, which leads to a low capacitance of the sensor element and thus necessitates electronics which are elaborate and therefore susceptible to faults.
Further elongated sensor elements which, in addition to the acquisition of a contact of the sensor element by means of a capacitive sensor area, monitor the approach of an object to the sensor element capacitively are known from JP 2008 130366 A and US 2005/0092097 A1.